1. Field of the Invention
The present invention relates to a solid-state electronic image sensing device with high subsampling efficiency and a method of reading a video signal out of the same. More particularly, the present invention relates to a solid-state electronic image sensing device of the type including a plurality of photosensitive cells arranged in a row and a column direction, vertical transfer paths adjoining the photosensitive cells in the row direction and in which vertical transfer electrodes are formed, and transfer gates for transferring signal charges accumulated in the photosensitive cells to the vertical transfer paths, and a method of reading a video signal out of the same.
2. Description of the Background Art
In a digital still camera, for example, including a CCD (Charge Coupled Device) image sensor or similar solid-state electronic image sensing device, image data with high quality should only be produced for being recorded in a recording medium. When the digital still camera is operated in an AE (Automatic Exposure) or an AF (Automatic Focus) photometry mode or in a shoot mode, it is not necessary to display a subject to be picked up on a monitor in the form of high quality image data.
So long as the image sensing device has a relatively small number of pixels, e.g., about 640xc3x97480 pixels, it may be driven by a conventional drive system without regard to the operation mode of the camera, i.e., the AE or AF photometry mode, the shoot mode or a mode for recording of a video signal representative of a subject in a recording medium. Today, the digital still camera with the image sensing device implements image quality comparable with one achievable with a silver halide photosensitive type of film. The number of pixels of such a camera is further increasing to meet an increasing demand for higher image quality.
The increasing number of pixels of a solid-state image sensing device, however, brings about a problem that when the device is driven by the same system at all times, signal processing for AE, AF and display of an image on a monitor cannot be rapidly completed within a preselected period of time. This prevents image data representative of a subject to be rapidly recorded in a recording medium, so that the operator of the camera is apt to miss a chance to release the shutter.
To cope with the increasing number of pixels, Japanese Patent Publication No. 6148/1985 discloses a high resolution, reliable image pickup system needing no additional optics and therefore obviating the deterioration of dimensional specification and yield at the production stage (Prior Art Document 1 hereinafter). The image pickup system of Prior Art Document 1 includes a four-phase drive, frame transfer type solid-state image sensing device. Specifically, a potential well is formed beneath a particular electrode in each field so as to pick up an object and read an image signal representative of the subject. As a result, a four field, one frame image is formed. At the same time, the duration of signal storage is controlled in accordance with a difference between the valid areas of transfer electrodes.
Japanese Patent No. 2660592 proposes a high-definition still camera with a monitoring capability that allows the operator to set a view angle while watching a subject and release the shutter of the camera at any desired time (Prior Art Document 2 hereinafter). The camera is therefore capable of dealing with a still picture in the same manner as a moving picture, e.g., a moving picture recorded by a Video Cassette Recorder).
Japanese Patent No. 2660594, like Prior Art Document 2, discloses an electronic still camera capable of reading out a signal on a four-field basis (Prior Art Document 3 hereinafter). The camera of Prior Art Document 3 executes, after exposure, read-out of stored needless signal charges during a single field scanning period and then reads out pixel signals by field scanning so as to obviate smear components. Further, after reading out the needless charges, the camera resumes scanning a field not shifted and sequentially reads out pixel signals field by field. This is successful to uniform the influence of dark current on the field-by-field pixel signals. Consequently, there can be obviated flicker ascribable to, e.g., irregularity in field-by-field luminance at the time of reproduction.
Further, Japanese Patent No. 2721603 teaches a solid-state image sensing device and a method of driving it (Prior Art Document 4 hereinafter). Generally, when a solid-state image sensing device and a monitor for displaying a scene being picked up are noticeably different in vertical resolution, vertical flicker, for example, appears on the monitor and degrades motion resolution while extending a processing time. The loss of power is another problem to arise in such a situation. The image sensor disclosed in Prior Art Document 4 reads, at the time of monitoring, charges out of only two kinds of photoelectric transduction elements alternately and reads a single frame over a 2V period so as to enhance motion resolution and obviate vertical jitter. At the time of reproduction, the device of Prior Art document 4 reads charges out of only one kind of photoelectric transduction elements and sets up the same saturation charge as in a still picture shoot mode, thereby making it needless to boost a drive voltage. This solves the power loss problem.
As stated above, a procedure for reading signal charges out of a solid-state electronic image sensing device has recently been devised in various ways in order to subsample or otherwise deal with signal charges in a manner matching with AE, AF or monitoring of a subject.
The system disclosed in Prior Art Document 1, however, has the following problems left unsolved because it effects four-phase drive at all times. Specifically, when color filters are arranged in a so-called Bayer pattern for color image pickup, the four-phase drive fails to cause all of three primary colors R (red), G (green) and B (blue) to appear in a single field. Subsampling effected in this condition would prevent a color image from being adequately displayed. In addition, the drive system is limited to four-phase drive, as mentioned above.
The devices of Prior Art Documents 2 and 3 each read out signal charges out of a great number of pixels in four consecutive fields constituting a single frame. To monitor a scene to pick up, use is made of one-half of the resulting image data, i.e., two fields of image data. However, neither Prior Art Document 2 nor 3 is practicable without resorting to a period of time corresponding to four fields in actually reading out the image data for monitoring. More specifically, during AE or AF photometry requiring rapid processing , the same period of time as during usual read-out is necessary. Prior Art Documents 2 and 3 therefore make no contribution to rapid processing.
The device of Prior Art Document 4 executes subsampling by reading out signal charges of two fields and discarding signal charges of the other two fields. The maximum degree of sampling available with Prior Art 4 is therefore one-fourth, limiting the signal reading rate.
As stated above, while various schemes have been proposed to subsample a video signal to be output from a solid-state image sensing device for realizing rapid signal processing, it is difficult to increase the degree of subsampling therewith.
It is therefore an object of the present invention to provide a solid-state electronic image sensing device capable of increasing the degree of subsampling more than the conventional devices, and a method of reading a video signal thereoutof.
In accordance with the present invention, a solid-state electronic image sensing device includes a plurality of photosensitive cells arranged in a row direction and a column direction. Vertical transfer paths adjoin the photosensitive cells arranged in the row direction and having vertical transfer electrodes formed thereon. Transfer gates each intervene between one photosensitive cell and the vertical transfer path adjoining it for transferring a signal charge accumulated in the former to the latter. Specifically, the transfer gates are made up of a first group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+1xe2x80x9d, an xe2x80x9cN+5xe2x80x9d and an xe2x80x9cN+13xe2x80x9d row (N being a positive integer), a second group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+2xe2x80x9d row, a third group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+3xe2x80x9d, an xe2x80x9cN+7xe2x80x9d, an xe2x80x9cN+11xe2x80x9d, and an xe2x80x9cN+15xe2x80x9d row, a fourth group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+4xe2x80x9d, an xe2x80x9cN+8xe2x80x9d, an xe2x80x9cN+12xe2x80x9d and an xe2x80x9cN+16xe2x80x9d row, a fifth group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+6xe2x80x9d, an xe2x80x9cN+10xe2x80x9d and an xe2x80x9cN+14xe2x80x9d row, and a sixth group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+9xe2x80x9d row. A particular gate pulse is simultaneously applied to each of the first group to the sixth group of transfer gates via a respective signal line.
Also, in accordance with the present invention, a method of reading out signal charges out of the a solid-state electronic image sensing device having the above configuration includes the steps of simultaneously applying gate pulses to a first group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+1xe2x80x9d, an xe2x80x9cN+5xe2x80x9d and an xe2x80x9cN+13xe2x80x9d row, simultaneously applying gate pulses to a second group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+2xe2x80x9d row, simultaneously applying gate pulses to a third group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+3xe2x80x9d, an xe2x80x9cN+7xe2x80x9d, an xe2x80x9cN+11xe2x80x9d and an xe2x80x9cN+15xe2x80x9d row, simultaneously applying gate pulses to a fourth group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+4xe2x80x9d, an xe2x80x9cN+8xe2x80x9d, an xe2x80x9cN+12xe2x80x9d and an xe2x80x9cN+16xe2x80x9d row, simultaneously applying gate pulses to a fifth group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+6xe2x80x9d, an xe2x80x9cN+10xe2x80x9d and an xe2x80x9cN+14xe2x80x9d row, and simultaneously applying gate pulses to a sixth group of transfer gates adjoining the photosensitive cells arranged on an xe2x80x9cN+9xe2x80x9d row. As a result, signal charges are sequentially read out of the first group to the sixth group of transfer gates.
Further, in accordance with the present invention, a solid-state electronic image sensing device for picking up a scene includes a plurality of photosensitive cells arranged in a row direction and a column direction each for transforming incident light to a corresponding electric signal by photoelectric transduction. Vertical transfer paths adjoin the photosensitive cells for transferring in a vertical direction signal charges read out of the photosensitive cells or devices. Transfer gates each intervene between one photosensitive cell and the vertical transfer path adjoining it for transferring a signal charged accumulated in the former to the latter at a preselected timing. A horizontal transfer path sequentially transfers the signal charges in a horizontal direction. A drive signal generating circuit feeds vertical drive signals, which include transfer gate pulses for driving the transfer gate at a preselected timing, to the transfer gates to thereby transfer the signal charges from the vertical transfer paths to the horizontal transfer path. The vertical transfer paths each have vertical transfer devices each two of which are assigned to a single photosensitive cell for thereby preventing the signal charges read out of the photosensitive cells from being mixed together. The drive signal generating circuit generates the vertical drive signals regularly for reading out the signal charges out of the photosensitive cells at the interval of 2i+1xe2x88x921 rows (i being a natural number) from preselected photosensitive cells of the same color as a first color, which is a characteristic as seen in the row direction, and reading out the signal charges out of the photosensitive cells at the interval of 2i+1xe2x88x921 rows from preselected photosensitive cells of the same color as a second color, which is a characteristic as seen in the row direction. The vertical drive signals are fed at the above intervals via signal lines arranged in the image sensing device.
Moreover, a method of reading signal charges representative of a scene picked up out of a solid-state electronic image sensing device having the above configuration includes the steps of forming on each of the vertical transfer paths vertical transfer devices each two of which are assigned to a single photosensitive cell for thereby preventing the signal charges read out of the photosensitive cells from being mixed together, and causing the drive signal generating circuit to generate the vertical drive signals regularly for reading out the signal charges out of the photosensitive cells at the interval of 2i+1xe2x88x921 rows from preselected photosensitive cells of the same color as a first color, which is a characteristic as seen in the row direction, and reading out the signal charges out of the photosensitive cells at the interval of 2i+1xe2x88x921rows from preselected photosensitive cells of the same color as a second color, which is a characteristic as seen in the row direction. The vertical drive signals are fed at the above intervals via signal lines arranged in the image sensing device.